# Canonical URL

An HTML tag that tells search engines which version of a page is the "master" copy when duplicate or near-duplicate content exists at multiple URLs.

A canonical URL is specified using the `<link rel="canonical" href="...">` tag in a page’s `<head>`. It signals to search engines which URL should be treated as the authoritative version when the same or very similar content is accessible at multiple addresses.

## Common Scenarios Requiring Canonicals

- HTTP vs HTTPS versions of a page
- Trailing slash vs non-trailing slash URLs
- URL parameters (e.g. `/product?color=blue` vs `/product`)
- Paginated series where the first page content overlaps with later pages
- Syndicated content published on multiple domains

## Self-Referencing Canonicals

It is best practice for every indexable page to include a self-referencing canonical — a canonical tag pointing to its own URL. This prevents accidental canonicalisation from scrapers or syndication.

## Canonical vs Noindex vs 301 Redirect

- **Canonical** — preserves the duplicate URL but consolidates ranking signals to the preferred version. Use when you want to keep both URLs accessible.
- **Noindex** — removes the page from the index entirely.
- **301 redirect** — permanently redirects one URL to another. The strongest signal; use this when the duplicate URL should never be directly accessed.